The use of active noise control (ANC) systems in automotive applications has been common practice for well over a decade. Many of these systems use a form of feedback control in which an error microphone, placed inside the vehicle cabin, provides feedback to the algorithm in order to assess the effect of the anti-noise signal as it attempts to cancel primary noise. Prior work pertaining to error microphone placement has been primarily limited to numerical studies based on wave propagation models with known primary sources and simply defined boundaries. The goal of this paper is to establish empirically-based metrics which can be used to quantitatively describe why one microphone position in the vehicle is superior or less favorable when compared to another. These metrics are used when considering a multiple-input multiple-output (MIMO) ANC system along with multiple primary noise sources whose location and propagation characteristics are unknown. Applications of these metrics become even more critical when using a common error microphone for multiple MIMO ANC systems running simultaneously. The application of these metrics will be shown in a passenger vehicle with multiple MIMO ANC systems running simultaneously trying to cancel both narrow band harmonic and broadband noise. Empirical correlation of these metrics will be shown with in-vehicle acoustic measurements as the ultimate performance objective of an ANC system is to reduce noise inside the cabin. These meaning of these metrics will also be examined when referenced to the filter update equation as these metrics improve the effectiveness of the ANC system as well as its stability.